Kievan Rus (also Kyivan Rus) was a medieval polity in Europe, from the late 9th to the mid 13th century, when it disintegrated under the pressure of the Mongol invasion of 1237–1240.

Contemporarily, the state was known as "land of the Rus'" (Old East Slavic русьскаꙗ землꙗ, from the ethnonym Рѹ́сь, Greek Ῥώς, Arabic الروس ar-Rūs), in Greek as Ῥωσία, later also Latin pseudo-antique Ruthenia. The name "Kievan Rus'" (Ки́евская Русь Kievskaya Rus’) was coined in the 19th century in Russian historiography to refer to the period when the center was in Kiev. In English, the term was introduced in the early 20th century, when it was found in the 1913 English translation of Vasily Klyuchevsky's A History of Russia, to distinguish the early polity from successor states, which were also called Rus in their title. Also in the 20th century, the Russian term was rendered in Belarusian and Ukrainian as Кіеўская Русь Kijeŭskaja Rus’ and Ки́ївська Русь Kyivs'ka Rus’, respectively.

The early phase of the state is sometimes known as the "Rus Khaganate", while the history of Rus' proper begins in 882, when the capital was moved from Novgorod to Kiev, after Varangians (Vikings), who were called Rus, liberated this slavic city from the Khazars' tribute. The state reached its zenith in the mid 11th century, when it encompassed territories stretching south to the Black Sea, east to the Volga, and west to the Kingdom of Poland and to the Grand Duchy of Lithuania. The reigns of Vladimir the Great (980–1015) and his son Yaroslav I the Wise (1019–1054) constituted the "Golden Age" of Kiev, which saw the introduction of Christianity and the creation of the first East Slavic written legal code, the Russkaya Pravda ("Justice of Rus").

Simon Franklin is Professor of Slavonic Studies at the University of Cambridge, UK. He is a Fellow of Clare College.

In 2007 he was awarded the Lomonosov Gold Medal by the Russian Academy of Sciences for outstanding achievements in research in Russian history and culture.